1. Field of the Invention
The present invention relates to nanoporous dielectric films and to a process for their manufacture. Such films are useful in the production of integrated circuits.
2. Description of the Prior Art
In the production of integrated circuits, the problems of interconnect RC delay, power consumption and crosstalk become more significant as feature sizes approach 0.25 xcexcm and below. The use of low dielectric constant (K) materials for interlevel dielectric and intermetal dielectric applications partially mitigate these problems but each of the material candidates having dielectric constants significantly lower than the currently employed dense silica suffer from disadvantages. Most low dielectric constant materials developments use spin-on-glasses and fluorinated plasma chemical vapor disposition SiO2 with K of  greater than 3. Some organic and inorganic polymers have dielectric constants in the range of about 2.2 to 3.5, however, these have the problems of low thermal stability, poor mechanical properties including low glass transition temperature, sample outgassing, and long term reliability questions.
Another approach has been to employ nanoporous silica which can have dielectric constants in the range of about 1 to 3. Porous silica is attractive because it employs similar precursors, for example tetraethoxysilane (TEOS) as used for spun-on glass (SOG""s) and CVD SiO2 and due to the ability to carefully control pore size and size distribution. In addition to low dielectric constant, nanoporous silica offers other advantages for microelectronics including thermal stability up to 900xc2x0 C., small pore size ( less than  less than microelectronics features), use of materials, namely silica and precursors that are widely used in the semiconductor industry, the ability to tune dielectric constant over a wide range and deposition using similar tools as employed for conventional spun-on glass processing.
High porosity leads to a lower dielectric constant than corresponding dense materials, and additional compositions and processes may be introduced as compared to a denser material. Materials issues include the need for having all pores significantly smaller than circuit feature sizes, the strength decrease associated with porosity, and the role of surface chemistry on dielectric constant and environmental stability. Density (or the inverse, porosity) is the key nanoporous silica parameter controlling property of importance for dielectrics. Properties of nanoporous silica may be varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to dense silica with a porosity of 0%. As density increases, dielectric constant and mechanical strength increase but the pore size decreases. This suggests that the optimum density range for semiconductor applications is not the very low densities associated with Kxcx9c1 but rather, higher densities which yield higher strength and smaller pore size.
Nanoporous silica films can be fabricated by using a mixture of a solvent and a silica precursor which is deposited onto a silicon wafer by conventional methods of spincoating, dip-coating, etc. The precursor polymerizes after deposition and the resulting layer is sufficiently strong such that it does not shrink during drying. Film thickness and density/dielectric constant can be controlled independently by using a mixture of two solvents with dramatically different volatility. The more volatile solvent evaporates during and immediately after precursor deposition. The silica precursor, typically, a partially hydrolyzed and condensed product of TEOS, is polymerized by chemical and/or thermal means until it forms a gel layer. The solvents are then removed by increasing the temperature. Assuming that no shrinkage occurs after gelation, the density/dielectric constant of the final film is fixed by the volume ratio of low volatility solvent to silica. EP patent application EP 0 775 669 A2, which is incorporated herein by reference, shows a method for producing a nanoporous silica film with uniform density throughout the film thickness. The preferred method for producing nanoporous dielectrics is through the use of sol-gel techniques whereby a sol, which is a colloidal suspension of solid particles in a liquid, transforms into a gel due to growth and interconnection of the solid particles. One theory is that through continued reactions within the sol, one or more molecules within the sol may eventually reach macroscopic dimensions so that they form a solid network which extends substantially throughout the sol. At this point, called the gel point, the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, the term xe2x80x9cgelxe2x80x9d as used herein means an open-pored solid structure enclosing a pore fluid.
For nanoporous silica, starting materials are silanes such as alkoxysilanes. Because of their high volatility and low viscosity, these precursors are first hydrolyzed with small amounts of water and optional acid catalyst under reflux conditions. At room temperature or below, these solutions have sufficient stability such that their shelf life is in excess of six months. In order to form a gel from this solution, additional catalyst and reactant (water) is added. The relative rates of hydrolysis and condensation are strongly dependent on pH. For hydrolysis, the rate is very high at pHxcx9c2 and decreases exponentially to a minimum at pHxcx9c7. At higher pH, the hydrolysis rate increases exponentially. For condensation, the rate is slowest at pHxcx9c2 and increases to a maximum at pHxcx9c8-9.
It has been possible to add catalyst/water shortly before deposition by mixing two fluids together or after deposition by vapor exposure. However, if the catalyst is added before deposition, deposition problems arise from the changing solution viscosity as the polymers grow and the fact that the catalyst/water must be added to a small amount of precursor for each wafer. The addition of catalyst and water via a vapor phase eliminates those problems but involves safety and equipment issues associated with handling volatile bases such as ammonia.
The invention provides a process for forming a nanoporous dielectric coating on a substrate which comprises the steps of:
(a) blending at least one alkoxysilane with a relatively high volatility solvent composition, a relatively low volatility solvent composition, and optional water to thus form a mixture having a pH of about 2 to about 5, and causing a partial hydrolysis and partial condensation of the alkoxysilane;
(b) adding a sufficient amount of a base to the result of step (a) to raise the pH of the mixture to about 8 or above;
(c) depositing the raised pH mixture resulting from step (b) onto a substrate while evaporating at least a portion of the relatively high volatility solvent composition;
(d) exposing the result from step (c) to a water vapor; and
(e) evaporating the relatively low volatility solvent composition.
The invention also provides a semiconductor device produced by the process which comprises the steps of:
(a) blending at least one alkoxysilane with a relatively high volatility solvent composition, a relatively low volatility solvent composition, and optional water to thus form a mixture having a pH of about 2 to about 5, and causing a partial hydrolysis and partial condensation of the alkoxysilane;
(b) adding a sufficient amount of a base to the result of step (a) to raise the pH of the mixture to about 8 or above;
(c) depositing the raised pH mixture resulting from step (b) onto a substrate while evaporating at least a portion of the relatively high volatility solvent composition;
(d) exposing the result from step (c) to a water vapor; and
(e) evaporating the relatively low volatility solvent composition.
It has now been found that a low dielectric constant nanoporous silica film can be produced by the post deposition step of exposing a deposited film to moisture, such as atmospheric moisture. In this procedure, a base is mixed into a sol in order to catalyze the gelation and aging reactions once a sufficient amount of water has been introduced into the system. The base may be in the form of an amine in order to maintain basicity while decreasing the volatility of the base.
This invention avoids the problems of adding catalyst shortly before deposition or in the vapor phase after deposition but still retains good shelf life. The invention performs the reactions in a particular order which is different than other sol-gel processes. According to the invention, the silane is first hydrolyzed by adding a substoichiometric amount of water and sufficient acid to maintain the pH of about 2 to 5 (conditions of rapid hydrolysis and slow condensation). The pH of the solution is then raised to a region in which the condensation rate is high (pH greater than 8). Although some condensation occurs, the solution will not gel as there is insufficient water. This solution is stable until deposition. During deposition and shortly thereafter, the film will rapidly absorb atmospheric water which will enable the deposited solution to gel. This process is aided by the cooling of the film precursor that occurs as a result of the rapid evaporation of the high volatility solvent. The gelled film then may be processed through a series of heating steps to remove remaining solvent and produce the low density nanoporous silica film. This process may be employed on either a patterned wafer if an insulator on metal approach is being used or on a plain wafer for a Damanscene process.
It would be desirable to produce a low density, low dielectric constant nanoporous silica by merely exposing a deposited film to moisture, such as atmospheric moisture.